As a geometrical structure for reducing the so-called nose-diving of a vehicle or the dropping of the front part of the vehicle when a sudden braking action has taken place, there are known several forms of anti-dive geometry in which the caster angle of the front wheels is increased as the wheel moves toward the bump condition (compressed state of the suspension system).
In order to accomplish such a dynamic change in the wheel alignment with a double wishbone suspension system in which upper and lower parts of the wheel carrier are coupled to the vehicle body each by way of a lateral A-arm, the rotational axial lines of the upper and the lower lateral arms with respect to the vehicle body are made to intersect each other at a point located behind the axle and above the point of contact between the wheel and the road surface or, in other words, the rotational axial line of the upper lateral link members with respect to the vehicle body is given with an upward inclination towards the front. Alternatively, the rotational axial line of the lower lateral link members with respect to the vehicle body is arranged in parallel with the longitudinal center line of the vehicle body while the rotational axial lines of the upper lateral link members of the right and the left suspension system with respect to the vehicle body are made to intersect each other ahead of the front axle line.
However, according to this concept, the points of pivotal attachment of the upper lateral arm with respect to the vehicle body must be offset from each other along the vertical or the lateral direction, and it may create such problems as producing a dead space in the engine room, making it difficult to ensure sufficient rigidity to the suspension structure due to the complication of the shapes of the mounts of the upper lateral arm, and so on.
As a suspension system for a steerable wheel which satisfies the requirements related to driveability and running stability of a vehicle and minimizes the space required to accommodate the motion of the wheel, there has been proposed a structure which, for instance as disclosed in Japanese patent publication No. 52-9889 (which is based on Austrian patent application No. 8125/68), have two pairs of lateral link members, one of the pairs connecting an upper part of the wheel carrier to the vehicle body while the other pair connect a lower part of the wheel carrier to the vehicle body. The wheel which is supported by means of such a linkage mechanism is steered about instantaneous centers of rotation as given by points of intersection of the lines each defined by connecting the pivot points of the corresponding lateral link member with respect to the vehicle body and the wheel carrier. Because such a suspension system offers a considerable freedom in design, it is possible to give a favorable dynamic property to the wheel alignment with appropriate geometric arrangement of the lateral link members and their pivot points. For instance, it is possible to change the caster angle of the wheel as the wheel undergoes a vertical motion so as to produce an anti-dive effect.
On the other hand, in case of a steerable wheel, since each wheel undergoes a relative vertical displacement also during a rolling motion of the vehicle, the force required to maintain a steering angle may change during a cornering action in a suspension system in which the caster angle changes according to the vertical motion of the wheel.
Further, a steerable wheel is typically steered by means of a tie rod extending laterally of the vehicle body and connected to a knuckle arm projecting from the wheel carrier along the fore-and-aft direction of the vehicle so as to transmit the torque applied to the steering wheel to the steered wheel by way of the lateral displacement of the tie rod. When the relationship between the knuckle arm which protrudes rearwardly and the tie rod connected thereto during a steering action is considered, one can see that the distance between the extension of the line connecting the two pivot points of the tie rod and the imaginary king pin center is closer for the inner wheel or, in other words, the knuckle arm of the inner wheel tends to form a toggle with the tie rod. This not only tends to cause an unfavorable change in the torque required to perform the steering action but also limits the maximum steer angle of the steerable wheel.
Yet further, a suspension system is generally required to have the capabilities to isolate the vehicle body from the vibrations arising from the irregularities of the road surface, and to ensure a driving stability by achieving a solid grip between the wheel and the road surface. It is therefore essential to achieve these two goals at as high levels as possible in designing a suspension system. Now, it is known that coinciding the imaginary king pin as given by the imaginary centers of rotation of the upper and lower parts of the wheel carrier with the center of the tire contact surface is effective in improving the capability of the vehicle to run straight ahead and reducing the force required to perform a steering action, and it is considered desirable to minimize the movement of the intersection between the imaginary king pin and the road contact surface during a steering action in order to reduce any irregular change in the force required to perform a steering action.
If the trajectory or the travel of the upper instantaneous center of rotation is made relatively long while the trajectory of the lower instantaneous center of rotation is made relatively short, for a given steering input, the position of the imaginary king pin during the steering action changes along a substantially conical surface created by a generatrix consisting of the imaginary king pin, and the travel of the trajectory of the intersection between the imaginary king pin and the road surface is minimized as discussed in a copending U.S. patent application Ser. No. 135,250.
Such a property can be obtained by appropriately selecting the spacing of the pivot points of the upper lateral link members with respect to the wheel carrier and the corresponding spacing of the lower lateral link members. As for the lower lateral link members, the fore-and-aft spacing of their pivot points at the wheel carrier is desired to be minimized. However, the pivot points consists of balls joints and, conventionally, it has not been possible to bring them sufficiently close to each other without causing an interference between them. As for the upper lateral link members, it is necessary to achieve a desired geometrical arrangement without reducing the mechanical strength of the wheel carrier. This is not always possible because a relatively large fore-and-aft and/or vertical spacing is required at a relatively upper part of the wheel carrier.